I've triggered this KASSERT on my dom0 twice in the last hour; once
with a very -current kernel, once with a kernel a few weeks old.
Happens with a lot of i/o activity in the domUs.
Anything I should get to help track down what's going on?

I guess both have xbdback_xenbus.c 1.46 ?
Can you try reverting it to 1.45 ?